1. Field of the Invention
The present invention relates to a castor and, more particularly, to a castor for a chair.
2. Description of the Related Art
A conventional castor for a chair in accordance with the prior art shown in FIG. 4 comprises a rotation body 10 provided with a connecting tube 11 pivotally mounted on a leg (not shown) of a chair, a shaft 12 extended through the rotation body 10, and two rollers 13 mounted on the shaft 12 and combined with the rotation body 10. However, the shaft 12 made of metal and the rollers 13 made of plastic material are made of different materials to increase the friction between the shaft 12 and the rollers 13 during rotation of the rollers 13, so that the rollers 13 are easily worn out during a long-term utilization. In addition, the rollers 13 are not rotatable on the shaft 12 smoothly.
Another conventional castor for a chair in accordance with the prior art shown in FIG. 5 was disclosed in U.S. Pat. No. 687,838 and comprises a rotation body 23 provided with a connecting tube 24 pivotally mounted on a leg (not shown) of a chair, a shaft 22 extended through the rotation body 23, two rollers 20 mounted on the shaft 22 and combined with the rotation body 23, and a plurality of rolling balls 21 mounted in the rollers 20 to facilitating rotation of the rollers 20. However, the rolling balls 21 made of metal and the rollers 20 made of plastic material are made of different materials to increase the friction between the rolling balls 21 and the rollers 20 during rotation of the rollers 20, so that the rollers 20 are easily worn out during a long-term utilization. In addition, the rolling balls 21 are made of metal so that the rolling balls 21 are easily rusted during a long-term utilization.